The present invention relates generally to brushless DC motors and, more particularly, to a speed control for a brushless DC motor wherein the speed of the motor is controlled by the voltage level of the power applied to it.
In a brushless DC motor, the rotor carries permanent magnets and requires no electrical connection. Cooperating electromagnets, which are repeatedly activated to drive the motor, are contained in a stationary armature. Power transistors are operated to activate the electromagnets in the proper timing sequence to drive the motor at a desired speed.
For sensing of the speed and position of the rotor of a brushless DC motor, stationary Hall effect devices are positioned adjacent to the rotor such that they are activated by the permanent magnets carried thereon. The Hall effect devices generate signals which are ultimately used to control the power transistors to generate correct timing of power pulses to the electromagnets. Logic circuitry associated with the motor receives the output signals from the Hall effect devices and processes the signals to turn on the correct power transitor at the correct time.
Usually, a desired speed is maintained in a brushless DC motor by pulse width modulation of power which is provided to the electromagnets. The power is provided as a voltage which is higher than that necessary to maintain the desired motor speed and then the pulse width is modulated such that the power delivered to the electromagnets is sufficient to maintain the desired speed of the brushless DC motor. Unfortunately, high frequency switching of the power to the electromagnets generates hysteresis and eddy current losses and electromagnetic interference (EMI). Since the motors frequently have leads of some length which serve as effective transmitters for the EMI. this is a particularly serious problem in equipment which is to be economically housed, for example in a plastic cabinet which provides no EMI shielding.
Consequently, there is a need for a method and apparatus of controlling the speed of a brushless DC motor which elminates the switching and resulting problems associated with pulse width modulation in the prior art speed controls for such motors.